Preparation of esters of oxodehydroabietic acid



peroxide. On dissolving this fraction in methanol and cooling to -20 C. a. yield of 21%, based on the hydroperoxide content of the oxidized ester, or methyl 9-hydroperoxydehydroabietate crystallized out. It had a melting point of 132.5-

Ten parts of this crystalline methyl Q-hydroperoxydehydroabietate was dissolved in 500 parts of methanol. The solution was agitated and a solution of 18 parts of ferrous sulfate dissolved in 400 parts of a 1:1 aqueous methanol mixture was slowly added at room temperature. Agitation was continued at room temperature for onehalf hour, after which the reaction mixture was heated at 50 C. and agitated at that temperature for 1.5 hours. The reaction mixture was then poured into volumes of water and the product extracted with ether. The ethereal extract was washed with water and dried with anhydrous sodium sulfate. The ether was removed by distillation and the residue containing 94% methyl 9- oxodehydroabietate, as indicated by its ultraviolet absorption spectrum, was distilled under reduced pressure. The fraction which distilled between 160-1'70 C. at 0.02 mm. pressure amounted to 8.9 parts and was crystallized from hexane. It was then recrystallized from aqueous methanol, whereby white crystals of methyl Q-oxodehydroabietate melting at 68-69 C. were obtained.

Example 3 Fifteen parts of a crystalline methyl 9-hydroperoxydehydroabietate was dissolved in 500 parts of methanol. The solution was agitated and with the temperature held at 10 C., 7 parts of ferrous sulfate dissolved in 1000 parts of a 25% aqueous methanol solution was added. The reaction mixture was allowed to come to room temperature and was then poured into 10 volumes or" water and the product extracted with ether and distilled as in the foregoing example. The product obtained amounted to parts. It was dissolved in aqueous methanol and crystallized, whereby the white crystalline methyl 9-oxodehydroabietate melting at 68-69 C. was obtained.

Example 4 Fifteen parts of ethyl dehydroabietate was oxidized with molecular oxygen in the presence of 0.75 part of benzoyl peroxide at 80 C. After 6.25 hours, the ester had absorbed 37.7 mole per cent of oxygen and the product was found on analysis to contain 40.3% of hydroperoxide.

One and one-half parts of this oxidate was dissolved in 50 parts of methanol. A solution of 2.0 parts of ferrous sulfate in a mixture of parts of water and parts of methanol was added to the agitated oxidate solution during a period of 1.5 hours. After heating at C. for 1.5 hours, the reaction mixture was poured into several volumes of water and the organic material was extracted with ether. The ethereal solution was dried and then evaporated to dryness. The product so obtained was shown by spectrographic analysis to contain 32-33% ethyl 9-oxodehydroabietate.

Any ester of S-oXodehydroabietic acid may be prepared in accordance with this invention by oxidizing the corresponding ester of dehydroabietic acid in liquid phase with oxygen and then reducing the hydroperoxide so obtained to the ketone. The esters of dehydroabietic acid which are so oxidized may be prepared from dehydroabietic acid my any of the methods commonly employed in the production of carboxylic acid esters as, for example, esterification of the acid with an alcohol under pressure or of the acid chloride with an alcohol or by heating an alkali metal salt of the acid with an alkyl halide. The dehydrcabietic acid which is esterified is readily obtained from a dehydrogenated or disproportionated rosin by solvent extraction. Instead of using the pure ester, an ester of commercial dehydrogenated rosin may be used or an ester of rosin may be dehydrogenated and used.

While the foregoing examples have shown the oxidation and subsequent reduction of the oxi dized product in accordance with this invention as applied to methyl and ethyl dehydroabietate, the process is equally applicable to any other alkyl, hydroxyalkyl, aryl, or aralkyl dehydroabietate as, for example, propyl, butyl, hydroxyethyl, glycerol, benzyl, etc., dehydroabietates.

The oxidation of the dehydroabietic acid esters to produce the 9-hydroperoxydehydroabietates is carried out by passing an oxygen-containing gas through the ester in the liquid state, either molten or dissolved in an inert solvent. Any gas containing free oxygen may be used for carrying out the oxidation as, for example, molecular oxygen or air. The temperature at which the oxidation is carried out will depend upon the reaction conditions. For example, if it is carried out in the absence of a solvent, the temperature must be at least that of the melting point of the ester being oxidized as, for example, above about 62 C. in the case of methyl dehydroabietate, etc. If a solution of the ester is used for carrying out the oxidation reaction as, for example, in tertiary butylbenzene, lower temperatures may be used for the oxidation. The maximum temperature at which the oxidation is carried out is that at which the hydroperoxide will decompose. In general, the oxidation is carried out at temperatures between about 60 C. and 130 C. and preferably between about 65 C. to about 100 (3., a particularly advantageous range being between about C. and C. The oxidation is usually carried out at about atmospheric pressure. However, if desired, super-atmospheric pressures may be used.

The oxidation of the dehydroabietic acid esters is preferably carried out in the presence of a free radical oxidation initiator; i. e., a material which undergoes thermal decomposition to form free radicals under the reaction conditions. Of particular value are the peroxidic free radical oxidation initiators such as organic peroxides and organic hydroperoxides which form free radicals under the reaction conditions. Exemplary of the organic peroxides which may be used to initiate the oxidation reaction are the acyl peroxides such as acetyl peroxide, benzoyl peroxide, etc., alkyl peroxides such as tert-butyl peroxide, methyl ethyl peroxide, etc. Exemplary of the organic hydroperoxides which may be used to initiate the oxidation reaction are the alkyl hydroperoxides such as tert-butyl hydroperoxide, and alkyl aryl and alkyl cycloalkyl hydroperoxides such as diphenylmethyl hydroperoxide, a,a-dimethylbenzyl hydroperoxide, a,adimethylp-isopropylbenzene hydroperoxide, methylcyclohexyl hydroperoxide, tetralin hydroperoxide, naphthene hydroperoxides, etc. When using the free radical oxidation initiators, an amount of from about 0.1% to about 20%, and preferably about 0.3% to about 10%, is added to the dehydroabietic acid ester being oxidized.

The oxidate so produced and containing the ester of 9-hydroperoxydehydroabietic acid may acme besubi ected-to-thereduction reaction-=acc6rdance with this invention ort-he 9-hydro'peroxy dehydroabietate may be separated fromtneoxidateand then reduced. ff-the purehydroperoxide is desired, it may be obtainedfrom-the crude oxidation-product, referred to the wit date, by countercurrent solvent extraction or any other extractive procedure. Examples-oi solvent combinations-"which may be used tor-the countercur-rent extractionare aqueous methanol, ethanol, 'di'oxan'e, etcs, solutions with'ialiphatic or alicyclic hydrocarbon solvents such a'shex ane, 'heptana-isooctane, cyclonexane, Pete.

The reduction of the" Q-hydroperOiiydehydm abietic acid ester is carried-out by contacting the-ester, preferably dissolved in Jan S01- vent, with a metallic reducingagent; Any mes tallic compound containing a metallic atom which capable of acting as a reducing agent;

i. -e., which exists in more-than one valence state 1' and is'in one of its lowerxvalenceafstatesand: is capable of beingoxidized-to one -cf its-higher valence states, may be employed in the process of this invention.- Such compounds'are design'a'ted herein and the claims appended "herewithasmetallic reducing agents. Exemplary oi the metallic.reducingcsagents whichxmay "be used are the salts suchas .th'e suliates; nitrates, chlorides, naphthenates, acetates, :etc., :of iron,

cobalt, nickel; noangan'ese,'. chromium, tin,= 1l'ead,

mercury, copper, etc, "wherein the metallic ion is in one of its lower oxidation-istates as,:for "example, ferrousrcobaltous, nickelous; manganous, stannous, =eu prou's, eto; salts. "Complexes of these metallic :ions.:may.likewise=be used. I II'he metallic reducing agent preferably: one which is water-soluble.

The process of this invention. :is .preie'rably practiced by .the gradual :additionwith agitation of a solution of the metallic reducing-agent to a solution of the .9-hydroperoxydehydroa'bietate or oxidate .cont'ainingi' this 'hydroperoxide. .-.Any inert solvent, may be T used; tor-carrying .out this reduction'reactionbut it is preferably a polar-sob vent; i. e., in which ionization is possible. 2 For example, the 9ehydroperoxydehydroabietatei or oXidate containing this. rhydroper'oxideimay be dissolved in methanol and .the metallic reducing agent then addedas a sol-utioninaqueous methanol. Any solvent in which both the :hydroperoxide andrr'netallicreducing agent-are soluble may be used as;.ior;example, aqueous ethanoLacetone,

dioxane, ;propanol; :isopropanol, .-.etc..- While. the preferredinethod is to carryout the reaction in a homogeneous system, it is possible to use a heterogeneous system," for example, by contacting a solution of the hydroperoxide in a solvent such as ether, benzene, toluene, 'etc., "with ana'qu'eous solution of the metal-salt. The of solvent used may be varied over a wide range butshould besufificient to insure that the reaction mixtureis homogeneous. Usually, the hydroperox-ide is dissolved in an amount of the solvent to obtain a solution of from-about l%-to=about 10% concentration. The amount of SOIVBIltTiIlrWhiCh' the metallic reducing agent is dissolved is likewise variable but need be no more than the amount necessary to dissolve the agent. As pointed out above, an aqueous methanol or aqueous ethanol solution is particularly effective as a solvent for the metallic reducing agent and usually is one in which the water content is from about to about 60%.

These 9-hydroperoxydehydroabietates are secondary hydroperoxides but, as pointed out above,

they da-"not readily undergo thermal 'decomp'oslition. However, when contacted witlra metallic reducing agent, the ketone is produced easily and in high yields. Just how. themetallic reducing agent functions 'to decompose these hydroperoxides to "the ketone is not known. If "a sufficient quantity ofthe metallic reducing agent is "added to provide one electron for each hydroperoxy radical; it maybe assumed that a straight oxidation-reduction reaction has occurred. However, if it were merely a case of themetalli'c ion acting as a iie'tlu'cingv agent, then any reducing agent-should be operable, but this ist-not the :case, for when the 9-hydroperoxydehydroabietates are contacted with-a nonmetallic reducing agent, the hydroperoxy radical is reduced "toxan alcohol groupto yields fl-hydroxydehydroabietater Furthermore, less than the amount of the metallic reducing agent necessary-to furnish one electron per-hydroperoxy group may be used andstill obtainfiithexketone. Thus, the reaction appears to bea :reductionreaoti'on in which the metallic ion functions as a catalyst for the. reduction reaction rather than asthe-actual reducing agent; Consequentlyythe amount of metallic reducing agent used in the process'of this invention may be varied from a catalytic "amount to an amount sufficient to provide one electron for each hydroperoxyradical :to bereduced to the keto group. Greater :amoun'ts Of'E'bh metallic reducing agent may: be used, if desired. Preferably, the amount of metallic :reducing agent employed is at least such as to-proyide: the. electron necessary to re duceithe "hydroperoxide. grou to the keto group. The solution of the metallic reducing agent is preferably'added itothe SOIUfiOIlnOf the hydro .peroxideiat a "slow: rate in order toavoid the pro-.- ductioniof a highly colored product. While any temperaturetrup .to-the thermal decomposition point ofthe alkyl hydroperoxydehydroabietate or slightlyabove may .beused, .there .is no particular advantagertobe realizedin carrying out the reac: tion at: an elevated temperature. Thereaction-is, therefore, -.normally carried out at about room t'emperature. -.I-Iowever, lower temperatures may be ,used 2.8,101' example-0 C. oreven lower, if the-solvent medium is such thatit remains liquid at temperatures below 0 C. In generaL-a temperature of from about .0C.=to aboutloil 6., and preferably from about .l0".C.to about 60 C1, is used.

The; esters of,.-9-oxodehydroabietic "acid ob.- tained-by. the .aboveedescribed reduction reaction are .most easily separated .from the reaction mixture-by pou-ring the latter into a large volume of water .asato dissolve the inorganic. salts, and thenrextracting the organic product by means of ether-v or some other water-immiscible v solvent. The product canthen lac-separated from the extractant by distillation. This crude product irequentlycontains colored bodies which are not easilyi removed lay-crystallization. Removal of thesecolored bodies-may usually be accomplished by distillingithecrudeproduct at reduced res sure or by chromatographing it as, for example, onan-alurnina column-or with any other adsorbent which is effective for the separation of such colored bodies. By then recrystallizing the product from which such colored bodies have been removed, it is possible to obtain the esters of 9-oxodehydroabietic acid in pure form.

That the ketone produced in accordance with this invention is a dehydroabietic acid ester substituted by a keto group in the 9-position has been shown by the fact that an ultraviolet absorption spectrum of the product indicates that the carbonyl group is in the conjugate position with respect to the benzenoid nucleus. The keto group must then be on either the 9- or I l-carbon atom of the dehydroabietic acid nucleus. That it is not in the le-position was shown by the fact that a negative idoiorm test was obtained with this product.

The esters of g-oxodehydroabietic acid produced in accordance with this invention by the oxidation of dehydroabietic acid esters and subsequent reduction of the hydroperoxide so obtained are useful as intermediates in the synthesis of other derivatives of dehydroabietic acid and particularly as intermediates in the synthesis of pharmaceuticals.

What I claim and desire to protect by Letters Patent is:

1. The process of preparing an ester of 9- oxodehydroabietic acid which comprises oxidizing an ester of dehydroabietic acid in liquid phase with a gas containing free oxygen in the presence of a free radical oxidation initiator at a temperature of from about 60 C. to about 130 C. and contacting the oxidate containing' the ester of 9-hydroperoxydehydroabietic acid which is so produced with a metallic reducing agent to form the said ketone.

2. The process of preparing an alkyl 9- oxodehydroabietate which comprises oxidizing an alkyl dehydroabietate in liquid phase with a gas containing free oxygen in the presence of a free radical oxidation initiator at a temperature of from about 60 C. to about 130 C. and contacting the oxidate containing the alkyl' Q-hydroabietate and contacting said hydroperoxide with a metallic reducing agent to form the ketone.

4. The process of preparing ethyl 9-oxodehydroabietate which comprises oxidizing ethyl dehydroabietate in liquid phase with a gas containing free oxygen in the presence of a free radical oxidation initiator at a temperature of from about 60 C. to about 130 C. to produce an oxidate containing ethyl Q-hydroperoxydehydroabietate and contacting said hydroperoxide with a metallic reducing agent to form the ketone.

5. The process of preparing methyl 9-0xodehydroabietate which comprises oxidizing methyl dehydroabietate in liquid phase with a gas containing free oxygen in the presence of a free radical oxidation initiator at a temperature of from about 60 C. to about 130 C., separating the methyl 9-hydroperoxydehydroabietate in substantially pure form from the oxidate, and contacting the said hydroperoxide with a metallic reducing agent to form the ketone.

6. The process of preparing methyl 9-oxodehydroabietate which comprises oxidizing methyl dehydroabietate in liquid phase with a gas containing free oxygen in the presence of a peroxidic free radical oxidation initiator at a temperature of from about 60 C. to about 130 C., separating the methyl 9-hydroperoxydehydroabietate in substantially pure form from the oxidate, and reducing the said hydroperoxide by contacting a solution of the hydroperoxide in an inert solvent at a temperature of about 0 C. to about 100 C. with a metallic reducing agent to form the ketone.

7. The process of preparing methyl 9-oxodehydroabietate which comprises oxidizing methyl dehydroabietate in liquid phase with a gas containing free oxygen in the presence of a peroxidic free radical oxidation initiator at a temperature of from about 60 C. to about 130 C., separating the methyl 9-hydroperoxydehydroabietate in substantially pure form from the oxidate, and reducing the said hydroperoxide by adding a solution of ferrous sulfate in an aqueous solvent to a solution of the hydroperoxide in a watermiscible inert solvent to form the ketone.

8. The process of preparing an ester of 9-ox0- dehydroabietic acid which comprises contacting an ester of 9-hydroperoxydehydroabietic acid with a metallic reducing agent.

9. The process of preparing an alkyl 9-0Xodehydroabietate which comprises reducing an alkyl 9-hydroperoxydehydroabietate with a metallic reducing agent.

10. The process of preparing methyl 9- oxodehydroabietate which comprises contacting methyl 9-hydroperoxydehydroabietate with a metallic reducing agent.

11. The process of preparing ethyl g-oxodehydroabietate which comprises contacting ethyl 9- hydroperoxydehydroabietate with a metallic reduoing agent.

12. The process of preparing methyl 9-oxodehydroabietate which comprises contacting methyl 9-hydroperoxydehydroabietate with a metallic reducing agent at a temperature of from 0 C.

' to about 100 C.

13. The process of preparing ethyl 9-oxodehydroabietate which comprises contacting ethyl 9- hydroperoxydehydroabietate with a metallic reducing agent at a temperature of from about 0 C. to about 100 C.

14. The process of claim 12 wherein the metallic reducing agent is ferrous sulfate.

15. The process of claim 13 wherein the metallic reducing agent is ferrous sulfate.

PAUL F. RITCHIE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,434,643 Drake Jan. 20, 1948 2,435,831 Harvey Feb. 10, 1948 OTHER REFERENCES Nystrom-Jr. Am. Chem. Soc.--vol. 69, May 1947, Pp. 1197-1199. 

1. THE PROCESS OF PREPARING AN ESTER OF 9OXODEHYDROABIETIC ACID WHICH COMPRISES OXIDIZING AN ESTER OF DEHYDROABIETIC ACID IN LIQUID PHASE WITH A GAS CONTAINING FREE OXYGEN IN THE PRESENCE OF A FREE RADICAL OXIDATION INITIATOR AT A TEMPERATURE OF FROM ABOUT 60* C. TO ABOUT 130* C. AND CONTACTING THE OXIDATE CONTAINING THE ESTER OF 9-HYDROPEROXYDEHYDROABIETIC ACID WHICH IS SO PRODUCED WITH A METALLIC REDUCING AGENT TO FORM THE SAID KETONE. 